The Robert Street Bridge is a reinforced concrete multiple-arch bridge that spans the Mississippi River in downtown St. Paul, Minnesota with the St. Paul Union Pacific Vertical-lift Rail Bridge crossing underneath it at an angle. It was built in 1924-1926 and was commissioned in the early 1920s to replace a wrought-iron span, originally built in 1884-1885, that had become obsolete due to increasing traffic. The engineers who designed the bridge had several obstacles to work around. The tracks of the Chicago, St. Paul, Minneapolis and Omaha Railway, St. Paul Union Depot, and the Chicago Great Western Railway mainline were factors. The engineers also had to provide adequate clearance above the river, as defined by the United States War Department at the time. Finally, the bridge had to clear Second Street in downtown St. Paul and work through a busy manufacturing district at the south end. The location of nearly every pier was dictated by these requirements. As a result, the bridge was designed with a combination of barrel-arch and rib-arch flanking spans and a rainbow arch for the central span.